Ceramic lamp envelopes with modern metal halide arc capsules have created a new class of metal halide lamp, see, for example, Geven, et al. in U.S. Pat. No. 5,424,609 and Carleton et al. in J. Ill. Eng. Soc. P139–145, Winter, 1996 (Proc. of IESNA Annual Conference). These lamps contain metal halide fill chemistries and two electrodes. A high voltage pulse between the electrodes is used to ignite the lamp. Normal current and voltage are then applied through the electrodes to excite the enclosed gas and fill materials to a plasma state. Typical fills include rare earth halides with various other additives, which can include thallium halide and calcium halide, in addition to an inert starting gas such as argon or xenon.
The ceramic arc tube is often jacketed in another envelope, called an outer jacket, to protect the inner arc tube from the atmosphere. Many of the lamp parts, especially the niobium in-leads, oxidize rapidly if exposed to air at the lamp operating temperatures, causing the lamp to fail. These outer jackets are usually thermally isolated from the arc tube by construction and contain a vacuum or are filled with a partial pressure of an inert gas and a getter material, for example, a zirconium and aluminum compound, to getter oxygen and hydrogen.
Often, the inner arc tube and outer jacket are mounted inside a parabolic reflector to gather and direct the generated light from the lamp in a useful beam pattern. This can be a flood or a spot beam for illumination of interior surfaces or building facades in exterior applications. Such lamps with halogen light sources are also commonly used for illuminating merchandise in stores and outside lighting in residential applications, for example, in security lighting. There is great interest in using ceramic metal halide lamps in the applications cited since they are efficient and provide excellent color rendering. The true colors of merchandise are rendered almost as if they were displayed in sunlight.
Economies of scale dictate using the same reflector for the new ceramic metal halide lamps (HCI lamps) as were used for halogen lamps. This keeps manufacturing costs to a minimum. It also allows the lamps to be used in existing fixtures.
Unfortunately, life tests have shown that the HCI lamps mounted in existing lamp structures fail prematurely at about 1500–200 hours, instead of progressing to their rated life expectancy of 10,00 hours. This is attributed to the rapid chemical attack by the fill material on the sealing glass (frit) used to make conventional HCI seals, (see Geven et al., supra). The problem is exacerbated when the lamps are run in the base up configuration, as they are used in many interior down-lighting applications. The seal is then subject to greater heat and therefore more active chemical reactions. To be a useful product in the markets mentioned, the lifetime of the lamp must be extended.
U.S. Published Patent Application No. 2003/0193280, published Oct. 16, 2003, and which is owned by the assignee of the instant invention, has attempted to at least partially solve the problem by interposing a light absorbing layer in the neck of the lamp, whereby extraneous light is converted to heat in the layer and then re-radiated in an unfocused manner with only a small portion of it being redirected to the seal area. The Publication suggests that the light-absorbing layer can be a black top coating on the neck interior or exterior. Alternatively, means can be provided during manufacture so that the neck portion is not metallized. While this procedure works for its intended purpose, it introduced other problems. For example, the solution is costly to implement and degrades lamp performance and appearance. The coating in the neck must be manually removed by mechanical or chemical means or, alternatively, a masking device must be incorporated into the neck area for the metallization process. Some of the light entering the neck of a normal lamp is reflected out of the lamp face and contributes to the total lumens. This light contribution is lost when the neck is made transparent. Painting the neck or using colored glass adds considerable cost to the lamp and substantially alters the appearance.